From John Latham, US National Center for Atmospheric Research and Alan Gadian, University of Leeds
Your article on geoengineering was balanced and wide-ranging (22 September, p 30). We would, however like to add several points to the idea of global cooling by cloud whitening or marine cloud brightening (MCB), which involves seeding maritime clouds with seawater droplets to make them reflect more sunlight.
MCB is not ineffectual vis-à-vis polar cooling: several modelling papers, by our team and others, show that it would create more cooling in polar regions than at lower latitudes, with substantial sea-ice restoration.
Such work also shows that MCB may reduce rainfall in regions that need rain, but by modifying the site of cloud-seeding, this may be avoided. There is no doubt that if MCB works as we think, the significant cooling would cause global rainfall reduction, but calculations suggest that virtually all of it will be over the oceans.
Whether we could seed clouds effectively on the scale required is unknown. If continued research yields only positive results, we would need field tests. This could be done over an area of about 100 kilometres, too small to have a significant effect on climate.
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Boulder, Colorado, US , Leeds, UK
